The present invention relates to a solid-state image pickup device.
In conventional CMOS image sensors, a plurality of photoelectric conversion parts (photodiodes) that become pixels are formed in a matrix manner in a front surface portion of a semiconductor substrate, and light is radiated (caused to become incident) to the photodiodes by being caused to pass through the gaps of interconnect layers, whereby the light is detected. However, because the aperture size of a photodiode with respect to a surface of incidence of light is limited by the presence of the interconnect layers, with the pitch of pixels miniaturized, it has been difficult to obtain sufficient utilization efficiencies of incident light.
There has been proposed a technique that involves providing microlenses on an interconnect layer in units of pixels and increasing sensitivity by collecting light on a photodiode via openings of the interconnect layer. However, with the trends toward high integration and scaledown of semiconductor devices growing stronger, it has become difficult to obtain sufficient sensitivity even by using microlenses.
To solve such problems, there has been proposed a back side radiation type image sensor which is such that a photodiode is irradiated with light from the back surface side (the side opposite to an interconnect layer) and high sensitivity is obtained by increasing the effective aperture ratio without the influence of the presence of the interconnect layer and the like.
Light that has passed through a color filter that allows light of a specific wavelength to pass is caused to become incident on the photodiode. In the color filter, three colors, for example, R (red), G (green) and B (blue) constitute one set. A ray of red light of long wavelength has a small absorption coefficient in single-crystal silicon compared to a ray of blue light of short wavelength and has a long penetration length. For this reason, light that has not been absorbed by a pixel for red color reaches a green pixel and a blue pixel, which are adjacent to the red pixel, thereby posing the problem of what is called mixing of colors in which a wrong color signal is given.
Mixing of colors is apt to occur in a peripheral part of a pixel matrix region having a large incident angle of light on a pixel. This mixing of colors is more highlighted in the back side radiation type with high aperture ratio capable of taking incident light in from the whole surface of a pixel. Also, in the back side radiation type, light (that has not been absorbed in Si) reflected from the interconnect layer positioned below the photodiode penetrates into an adjacent pixel and this may cause mixing of colors.
As described above, conventional back side radiation type image sensors as described above had the problem that mixing of colors is apt to occur.